Strip line and microstrip techniques are increasingly being used in microwave circuits to provide well characterized transmission line conductors that can be used to interconnect discrete circuit elements and to perform various impedance transformation functions. Both techniques, however, suffer from a variety of drawbacks.
Microstrip is generally characterized by a planar transmission line conductor spaced above a conducting ground plane. The impedance and velocity factor of the transmission line so formed is determined by factors such as the dielectric characteristics of the surrounding materials, the width of the planar conductor and its spacing from the ground plane.
In free space, microstrip works well. In actual application, however, its operation is sometimes impaired by stray coupling between the transmission line conductor and nearby objects. Fringing of the usual electromagnetic fields that extend above the conductor to foreign objects introduces irregularities into the impedance and velocity factor of the line, with a consequent undesirable effect on circuit performance.
In contrast to microstrip, strip line is generally characterized by a planar transmission line conductor disposed between two ground planes. Such construction offers a significant advantage over microstrip in that the problem of stray coupling to nearby objects is avoided. The second ground plane, which is omitted in microstrip construction, shields the transmission line conductor from the effects of nearby bodies and serves to confine the electromagnetic fields to the region between the two ground planes.
Although strip line is preferred because of its superior performance, microstrip is more often used because of practical considerations. First, microstrip circuitry is easier to fabricate. A conventional double sided printed circuit board can be formed into a microstrip circuit board by etching one side to form the transmission line conductors and leaving the other side unetched to serve as the ground plane. Furthermore, components such as transistors, resistors and capacitors can readily be soldered to the exposed microstrip transmission line conductors in order to form the desired electrical circuit. Strip line is cumbersome in comparison.
To construct an electronic circuit with strip line techniques, the desired transmission line conductors must first be sandwiched between two ground planes. This is normally done by fabricating a custom laminated assembly. The components must then be connected to the various transmission line conductors. This step is complicated by the fact that the transmission line conductors are, by necessity, isolated between the two ground planes. While insulated conductors extending up or down through a ground plane can be employed to connect external components to the embedded transmission lines, the attendant circuit complexity is substantial. The manufacture of a strip line circuit is thus significantly more difficult than that of a corresponding microstrip circuit.
In view of the practical difficulties associated with strip line, microstrip is used for the majority of microwave circuit applications. In some applications, however, even a small amount of stray coupling between a transmission line conductor and a foreign body, as might be expected to occur in any microstrip contruction, is unacceptable. One such application is in quadrature directional couplers.
In a directional coupler, power is coupled from a primary transmission line to a secondary transmission line by bringing the center conductors of these two lines sufficiently close to cause interaction of the transmission lines' electromagnetic fields. The end of the secondary transmission line adjacent the primary transmission line input is termed the coupled output. The opposite end of the secondary line is terminated in a matched load and is termed the isolated port. In operation, a known fraction, usually half, of the energy flowing in the forward direction of the primary transmission line appears at the coupled output. The other half of the energy appears at the far end of the primary transmission line, termed the direct output. In a quadrature directional coupler, the signals at the coupled and direct outputs are 90 degrees out of phase.
A system that employs a quadrature directional coupler is usually dependent, for proper operation, on precise matching of the magnitudes of the two outputs and a precise 90 degree phase shift. Such performance is usually not obtainable in a microstrip coupler because stray coupling to the transmission line conductors often causes an imbalance between the magnitudes of the output signals or a deviation from the desired quadrature phase relationship. In order to obtain satisfactory performance, microstrip couplers are generally not formed as an integral part of a system's main circuit board but are instead placed in a shielded enclosure and are connected to the circuit board by coaxial cables.
Even when a microstrip coupler is shielded to protect it from the effects of stray coupling, several operational defects persist. One is the problem of controlling the electromagnetic coupling between the two transmission line conductors to within a tight tolerance. When the coupling coefficient required between the transmission lines is high, the transmission line conductors must be spaced very closely. Because of this close spacing, any slight deviation presents a very great variation in coupling and a consequent imbalance between the magnitudes of the two outputs. Although photolithographic circuit board etching techniques are constantly improving, it is still difficult to obtain the requisite degree of accuracy in spacing required by microwave devices.
As still a further defect, microstrip couplers suffer from dispersion of the electromagnetic waves propagating along the coupled transmission lines. The signal traveling down the planar microstrip conductor is accompanied by a surrounding electromagnetic field. On one side of the conductor, this field travels in the dielectric region between the conductor and the ground plane. On the other side, however, this field travels in air. The velocities of these two waves are different, resulting in a dispersion of the incident signal into two phase-shifted signals by the time the wave has traveled to the end of the conductor.
As a final drawback, mounting a microstrip coupler in a shielded enclosure requires that coaxial cables be employed to connect the coupler to the main system circuit board. This increases the cost and complexity of a system and reduces its reliability.
The above-noted drawbacks apply to a variety of microstrip transmission line circuits other than quadrature directional couplers. For example, any type of transmission line coupler or transmission line phase shifter would be so affected.
U.S. Pat. No. 4,375,054 to Pavio is illustrative of microstrip couplers. A pair of transmission line coupling elements are formed on one side of a circuit board and a ground plane is provided on the other. The circuit assembly is mounted in a shielded enclosure to minimize stray coupling. External connections are made to the coupler through coaxial bulkhead connectors on the enclosure.
In discussing alternative coupler constructions, Pavio briefly reviewed strip line couplers which have two transmission lines embedded in an insulating material, sandwiched between two outer ground planes. Pavio concluded, "The structure is bulky, costly and difficult to incorporate with other microwave circuitry."
U.S. Pat. No. 4,150,345 to Goldman et al. shows another microstrip coupler. In discussing alternative constructions, Goldman, too, dismissed strip line, noting that the transition from microstrip to strip line "is relatively costly and mechanically unreliable."
Strip line couplers, such as were maligned by Pavio and Goldman, fall into two broad classes: edge-coupled and broadside-coupled. In the edge-coupled variety, the two transmission lines are formed adjacent one another in the same plane. (Virtually all microstrip couplers are of this type). In the broadside-coupled variety, the two transmission lines are formed one above the other, separated by an insulating layer. In both varieties, the coupled transmission lines are disposed between and insulated from the upper and lower ground planes.
Examples of strip line couplers are shown in U.S. Pat. Nos. 3,568,098 to Gerst and 4,375,053 to Viola et al. Gerst shows an edge-coupled coupler. Viola shows a variation on a broadside-coupled coupler. Connections to the Gerst device are made by bulkhead coaxial connectors. Connections to the Viola device are not discussed.
Yet another strip line coupler, this one a ring coupler, is shown in U.S. Pat. No. 4,127,832 to Riblet. Connections to the device are again made by bulkhead coaxial connectors.
From the foregoing, it will be recognized that high performance couplers are generally constructed as discrete circuit subsystems and are connected to the larger systems in which they are employed by coaxial cables. This is the case both with microstrip devices that are mounted in shielded enclosures and with existing strip line devices. This design philosophy increases substantially the cost and complexity of microwave systems and degrades their performance.
The present invention is directed to an improved form of strip line construction that allows strip line circuitry to be packaged in component form so that it can be soldered directly to a circuit board, just like any other component.
Thus, it is one object of the present invention to provide a coupler that can be soldered directly to a circuit board.
It is another object of the present invention to provide, more generally, an improved transition between microstrip circuitry and strip line circuitry.
It is still another object of the present invention to provide a strip line coupler that is inexpensive to manufacture.
It is yet another object of the present invention to provide a directional coupler that can be fabricated using conventional multilayer printed circuit board techniques.
It is still another object of the present invention to construct a strip line circuit component that obviates the need for an edge plating step in fabrication.
It is yet another object of the present invention to provide a directional coupler component that is mechanically and thermally stable.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description, which proceeds with reference to the accompanying drawings.